1. Field of the Invention
This invention relates to a magnetic recording and reproducing method and apparatus such as a VTR, and particularly to a video signal processing method and apparatus for correcting the disturbance of the timebase of a video signal and compensating the video signal for a lost part of the video signal due to dropout or the like, so as to stably achieve a good reproduced picture.
2. Description of the Related Art
When an image is reproduced on a magnetic recording and reproducing apparatus such as a VTR, a change of the position of a signal detecting medium such as a magnetic head relative to a recording medium such as a magnetic tape from the position thereof upon recording will cause the timebase of the reproduced video signal to vary. In this case, a small change of the timebase will cause a jitter in the reproduced picture, while a large change thereof will cause a skew, thus greatly affecting the stability of the reproduced picture.
Moreover, if the recording medium is scratched or soiled with dirt, the reproduced radio frequency (RF) signal is partially lost or reduced (or a so-called dropout is caused), thus considerably deteriorating the quality of the reproduced picture.
A method for solving the above problem is described, for example, in the paper "VTR Technology", chapter 6, pp. 107-133, in Vol. 5 of the Broadcasting Technology Series published by Nippon Hoso Publishing Association, May 1983, and employs a video signal processing apparatus comprising a timebase disturbance compensation circuit and a dropout compensation circuit as shown in FIG. 2. FIG. 2 is a block diagram of a conventional video signal processing apparatus.
Referring to FIG. 2, there are shown an input terminal 10 to which a video signal having a timebase disturbance is applied, an output terminal 20 at which a video signal corrected for the timebase disturbance and compensated for dropout is produced, and an input terminal 30 to which is applied a dropout signal indicating a dropout period in which the amplitude of the reproduced radio frequency (RF) signal is found to be considerably reduced when the envelope thereof is detected in the FM system of the VTR. In addition, there are shown an A/D converter circuit 1 for converting an input video signal into a digital signal, a video signal processing main memory 2 which is formed of a RAM or the like and functions as a timebase disturbance correcting buffer memory, and a dropout signal memory 3 formed of a RAM or the like. Shown at 7 is a horizontal synchronizing signal separation circuit. The horizontal synchronizing signal which has a timebase disturbance and is extracted from this horizontal synchronizing separation circuit 7 is supplied to a write clock generating circuit 50 and then to a write address control circuit 60.
The write clock generating circuit 50 generates a write clock pulse in synchronism with the horizontal synchronizing signal so as to coincide in timing with the input video signal having a timebase disturbance, or variation which is applied to the terminal 10. The write address control circuit 60 generates a write address based on the write clock pulse.
Thus, the video signal having a timebase variation from the terminal 10 is sequentially converted to a digital signal in synchronism with the write clock pulse by the A/D converter circuit 1, and then written in the video signal processing main memory 2 in accordance with the write address. Thus, since the video signal at a position corresponding to the clock pulse generated in synchronism with the horizontal synchronizing signal is sequentially recorded in a proper address of the main memory 2, the timebase disturbance, or variation of the recorded video signal can be corrected. At the same time, the dropout signal applied to the terminal 30 is written in the dropout signal memory 3 at the write address in synchronism with the video signal.
On the other hand, a stable reference synchronizing signal with no timebase variation is supplied via a terminal 40 to a read clock generating circuit 80, which then generates a read clock pulse in synchronism with the reference synchronizing signal. A read address control circuit 70 produces a read address in synchronism with this read clock pulse.
The video signal data stored in the video signal processing memory 2 is sequentially read from the read address at each horizontal scanning period, and supplied to a switching circuit 4. The dropout signal stored in the dropout signal memory 3 is read from the read address in synchronism with the video signal data. The switching circuit 4 is operated by the dropout signal read from the dropout signal memory 3 so as to select the output from a 1 H delay circuit 5 (H is the horizontal scanning period) during the dropout period and to select the output from the video signal processing main memory 2 during the other period. The 1 H delay circuit 5 is formed of a line memory or the like, and responds to the read clock pulse to delay the video signal data from the switching circuit 4 by 1 H. Therefore, when there is no dropout, or in a normal state, the same video signal as the contents of the video signal processing main memory 2 is produced from the 1 H delay circuit 5. When there is a dropout, the video signal data 1 H before is produced instead of the video signal having the dropout, or the video signal compensated for the dropout is produced from the 1 H delay circuit 5.
The video signal data produced from the 1 H delay circuit is supplied to a D/A converter circuit 6 where it is sequentially converted to an analog signal in synchronism with the read clock pulse. As a result, the video signal corrected for the timebase variation and compensated for the dropout is produced at a terminal 20.
The write clock generating circuit 50, as described in the above cited literature, is conventionally formed of an AFC (automatic frequency control) circuit operating on the basis of the horizontal synchronizing signal.
There is known a clock generating circuit formed of an APC (automatic phase control) circuit using the so-called burst signal included in the horizontal blanking period. In addition, there is also known a method of generating a write clock synchronized with the input video signal by use of the AFC circuit based on the horizontal synchronizing signal and the APC circuit based on an input video signal.
In the above prior art, the timebase variation is corrected by the video signal processing main memory 2 and so on, and the dropout is compensated for by the dropout signal memory 3, the 1 H delay circuit 5 (line memory) and so on independently from the timebase variation correction. Thus, the signal processing is complicated and hence needs a large-scale circuit arrangement.
When a dropout occurs so that a part of the video signal is lost, the dropout correction and the timebase variation compensation are made as described above, but a problem still remains as will be described below.
FIG. 3 shows the operation of the conventional video signal processing apparatus. As illustrated in FIG. 3, when a dropout including a synchronizing information part occurs, the video signal with a part lost by the dropout as indicated by DOC is compensated for the part by the application of the 1 H-before-video signal based on the dropout detecting signal as mentioned above. A part of the first line of the video signal which is not lost by the dropout and a faultless part of the third line and the following lines of the video signal (indicated by TBC in FIG. 3) are corrected for the time-base variations of the lines on the basis of the horizontal synchronizing signal HS1, HS3, . . . at the head of each line. However, since the horizontal synchronizing signal HS2 at the beginning of the second line is lost by the dropout, a write clock pulse precisely following the timebase variation of the horizontal synchronizing signal HS2 cannot be generated for the second line (indicated by NO-TBC) a part of which is not lost by the dropout. Thus, the timebase correction cannot be completely performed. This follows that the reproduced image within this line (indicated by NO-TBC) appears shifted in the horizontal direction on the screen, or that the reproduced picture quality is deteriorated.
Also, even when only the horizontal synchronizing signal HS2 at the head of the second line is lost by the dropout as illustrated in FIG. 4 which shows another operation of the conventional video signal processing apparatus or even when the horizontal synchronizing signal HS2 is not correctly separated, not due to the dropout, but due to a noise entering into the video signal as illustrated in FIG. 5 which shows still another operation of the conventional video signal processing apparatus (as indicated by NO-TBC in FIGS. 4 and 5), the reproduced picture quality is similarly deteriorated. Thus, the prior art has the drawback that when a part of the video signal is lost due to a dropout or other causes, no consideration is made for the necessary video signal processing on the lack of the horizontal synchronizing signal or burst signal which becomes the reference for the correction of the timebase variation, resulting in the deterioration of the reproduced picture quality.